Polyolefins such as polyethylene and polypropylene and other polymers may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, feed materials such as olefin, monomer and catalyst are introduced to a reaction zone, and a slurry comprising solid polyolefin particles in a liquid medium is formed in the reaction zone.
In commercial loop reactors, the various feed materials may be introduced to the loop reaction zone in various ways. For example, the monomer and catalyst may be mixed with varying amounts of a diluent prior to introduction to the loop reaction zone. In the loop reaction zone, the monomer and catalyst become dispersed in the slurry. The slurry circulates through the loop reaction zone, and the monomer reacts at the catalyst in a polymerization reaction. The polymerization reaction yields solid polymer particles in the slurry.
In order to recover the polymer solids from a particle form polymerization process, it is necessary to separate the polymer solids and hydrocarbon fluids that make up the effluent slurry withdrawn from the reactor. Typical separation systems include a reduction in pressure so that the hydrocarbon liquids vaporize. The vaporized hydrocarbons exit at a top portion of a flash gas separator. The polymer remains solid, and is recovered through a bottom portion of the flash zone. Other polymer recovery processes utilize multiple stage flash systems. For example, a first intermediate pressure flash zone and a second lower pressure flash zone may be employed. The temperature and the pressure in the first flash zone are such that a major amount of the diluent will be vaporized and that vapor can be condensed without compression by heat exchange with a fluid having a temperature, for example, in the range of from about 120° F. to about 200° F. The polymer particles from the first flash step may then be subjected to a lower pressure flash step to vaporize additional remaining diluent.
In a polymer recovery system utilizing one or more flash chambers, it has been common to have some diluent entrained in the polymer leaving the flash chamber(s). It is desirable, however, to obtain polymer product substantially free of diluent. If the polymer leaving the flash chamber contains an excessive amount of diluent, the polymer product can become tacky, and then may plug up lines and cause restrictions. In many polymer production processes, polymer solids after the flash chamber are subjected to further processing to remove residual and entrained diluent. Examples of such processing include purge zones, conveyer dryers, and other drying systems known in the art.
One method for removing additional amounts of diluent after the flash chamber involves passing the polymer solids through a purge column, wherein a non-combustible gas is used to remove the diluent. In the purge column, the non-combustible gas (nitrogen is often used) typically flows upward through downward-flowing polymer solids in the column. The purge gas is used to remove and carry away hydrocarbon fluids that remain with the polymer solids after the flash separating steps. The purge gas is especially suited to remove hydrocarbon fluids that are entrained in the pores and interstices of the solid polyolefin particles. At the top (or gas outlet) of the purge column, a fluid stream comprising the purge gas and removed hydrocarbon fluid (typically and preferably gas) exits.
At the bottom (or solids outlet) of the purge zone, polymer solids exit and are passed along for finishing operations, such as extrusion. Typically polymer solids are passed to an extruder feed tank which holds the polymer solids and feeds the polymer solids to the extruder. The extruder feed tank is at very low pressure (for example, 0 to 5 psig) and the polymer solids are fed by force of gravity to the extruder. Typically, the polymer solids have been passed from the purge zone to the extrusion feed zone via pneumatic transfer. Generally, the inert gas systems for the purge column and the extruder feed tank have been separate, since inert gas exiting from the purge column contained hydrocarbon impurities even after passing through a recovery unit to remove hydrocarbons.